Automatic draft control



2 1940. N. CUNNlNGHAM AUTOMATIC DRAFT COII'I'TROL Filed March 11, 1940 2Sheets-Sheet 1 InJen or Me] amm lyfiam a! *QM Aw 1940. N. CUNNINGHAMAUTOMATIC DRAFT CONTROL Filed March 11, 1940 2 Sheets-Sheet 2 [V06][21%22222 QM Patented Oct. 22, 1940 PATENT OFFICE AUTOMATIC DRAFTCONTROL Noel Cunningham, East Dundee, Ill, asolgnor to Illinois Iron &Bolt Company, Garpentersville,

111., a corporation of Illinois Application March 11, 1940, Serial No.323,365

Claims. (01.236-45) In the firing of solid fuels it is impossibletomaintain at all times uniform fuel bed textures and, as the texturesof fuel beds vary, so does the air permeability of such fuel beds. l5Where forced draft is used, it is customary to maintain uniform the rateof rotation of fans used in producing the forced draft and, at anyparticular rate of rotation, volume of air varies in inverse relation tothe static back pressure 20 against which such fans are required todeliver. When air permeability of any fuel bed increases, the staticback pressure against the I fan decreases and, unless fan output iscontrolled by manual or automatic means, the volume of air- 30combustion within the fuel bed to be reduced...

Ether of these conditions is undesirable.

It is not practicable to control closely the proper rates of airdelivery from fans used in forced draft firing by manual regulation anda 35 variety of automatic means are utilized for the.

purpose. These consist of devices which respond, either directly orindirectly, to changing static back pressures against which fanscurrently operate. In one type of such controls, 40 impulses fromvarying static pressures within the duct leading from the fan to thewindbox below the fuel bed provide the means for regulation. Thesepressure variations act upon pressure responsive mechanisms which inturn operate 45 dampers to effect the desired control.

The regulating member of an automatic air control is customarily abutterfly damper, that is, one which opposes to the flow of air equalsurfaces on either side of an axis about which it may 50 rotate. Suchbutterfly dampers may be either single blades effective over entire ductcrosssections or multiple blades, each centrally balanced andinterconnected to act in unison. To properly position such dampers theremust be 55 provided an actuating means which is responsive to changesinstatic back pressures which occur in the windbox below the fuel bed.

In forced draft firing, the periods during which the fan is inoperativeare herein referred to as off periods while the periods during which the5 fan is supplying forced draft are'referred to as on periods. During onperiods in any forced draft system, the true static back pressure of afuel bed which is receiving adequate air for combustion is the resultantof fan pressure and fuel bed air permeability. When an on period. occursafter an off period there should be an immediate build-up of staticpressure below the fire and this necessitates a flow of air from the fan";-to the windbox. Without such fiow of air the actuating member willnot receive sufllcient impulse to cause proper response and there willnot occur a correct setting tofuel bed requirements of the regulatingmember of an automatic control.

The proper functioning of an automatic air control is two-fold: itshould assure minimum fuel bed combustion during off periods and alsocorrectly proportion air delivery during on periods. In the past it hasbeen found that, if the regulating member of an automatic air controlwas effective in minimizing air fiow to the fuel bed during offperiods'it failed to allow the passage of sufficient air to developagainst the actuating medium the true static back pressure representedby the depth and air permeability of the fuel bed. It has heretoforebeen impossible to provide an air regulating member to be both effectivein minimizing air flow during off periods and also to allow air pressuresufficient to cause a proper build-up of static back pressure when onperiods occur.

Since'optimum conditions both during on and off periods could not besecured and, as correct air proportionment during on periods is the more40 important of the two, it has been customary to l design theregulating members of such automatic air controls to fit loosely eventhough in so doing much of their effectiveness during off periods issacrificed.

Coal is the normal fuel used where forced draft firing is practiced. Airpermeability of fuel beds is chiefly determined by coal c'aking andcoking -characteristics. Some coals form coke which is hard andrelatively air impermeable whereas the coke which forms when anothertype of coal is used is soft and readily air permeable. Still othercoals form a surface cake during off periods and, until this cake isbroken, it is resistant to passage of air.- Due to variations in fuelbed characteristics of coal from various producing areas, it isnecessary that a control system have means for adjustment and regulationin order to properly meter air in forced draft The primary object of myinvention is to provide an automatic air control having a close fittingregulating damper which will allow a minimum of air passage during 01!periods but will permit immediately an "on period operation occurs, astream of air to be directed against a lifting member to preliminarilyopen the damper damper to increase or decrease its lifting eifect. Stillanother object is to provide such a lifting member that also functionsas a counter-weight to oppose the full opening of the damper'dependingupon its position relative thereto.

A further object is to provide an adjustable stop member for limitingthe damper opening.

Further objects and advantages will be apparent from an examination ofthe following detailed description.

The full nature of my invention will be understood from the accompanyingdrawings and the following description and claims.

Fig. l is a sectional view showing a forced draft firing systemembodying my automatic air control.

Fig. 2 is an enlarged sectional view of the air control shown in Fig. 1during off operation."

Fig. 3 is a sectional view showing the air control in initial openingposition during on operation.

' In the drawings, l indicates a furnace or boiler, 2 a fan controlledby thermostatic means (not shown) as is well known in the art, I is anair inlet opening to the fan through which it draws the air which itmoves to the wind-box below the fuel bed and 4 is an air ductleadingfrom the fan to the wind-box. The air control is shown attachedto the air duct; it is positioned between the fan and the wind-box andcomprises generally a pressure responsive actuating member 5 and aregulating member 6 operably controlled by member 5.

In detail the actuating member 5 comprises a pressure responsive plateI; pivotally mounted in a housing 8, the free end of which movesadjacent the curved plate 9; housing 8 is open into the air duct fromthe bottom of plate 9 to wall of the housing toward the wind-box and isvented to the atmosphere by a fixed opening ll adjacent the upper end ofplate'9. Housing 8 is removably secured to the air' duct 4 by means ofscrews to. A stop member l2 comprising a bolt Ila and nut |2b isadjustably fastened in a curved slot 13 in a side wall of housing 8 tocontrol the maximum upward swing of the free end of plate I which inturn controls the maximum opening of damper i4.

The regulating member comprises a butterfly damper I 4 pivotally mountedwithin the duct 4 and adapted to effectively close said duct. Secured todamper l4 below the point of pivot on the wind-box side of the damper isa rigid extension IS. A connecting am it is pivotally joined to plate Iat itsupper end and to extension It at its lower end to operably jointhe pressure responsive plate I with damper I4. A swinging plate l'lprovides a closure for an air passageway ll positioned in damper '14;said plate being of suflicient weight to close opening it during oifperiods" and to be moved to open position during on periods." A liftingblade 20 having a rigid arm II is adjustably attached by a screw and nut22 to the end IQ of the extension l5, lifting blade 20 being positionedto receive the force of the Jet of air from air passageway 18, theresultant lifting effect being conditioned upon the angularity of blade20 in the air Jet. It will be apparent that during on periods of forceddraft firing the extent to which damper ll opens will determine theamount of air which the fuel bed will receive from the fan and that theopening of this damper is controlled by the static back pressure of thefuel bed exerted against the pressure responsive plate I in combinationwith the lifting effect exerted by the lifting blade 20, the maximumopening of damper It being determined by the positioning of the stop l2.

In operation the position of the elements of the air control as shown inFig. 2 is for of! periods" of forced draft operation, during which thepassage of.air through duct 4 induced by natural draft is minimized bythe close fitting of damper II and swing plate H. v However, when an "onperiod" occurs, immediately the air moved by the fan will strike platel1 causing it to open and the stream of air moving through passageway IIwill contact blade 20 to exert a lifting force on it, which willresultantly open damper H to admit more draft to the fuel bed. At once are slstance is encountered dependent upon the permeability of the fuelbed. This resistance produces a static pressure in the furnace side ofconduit 4 of definite value. This pressure will actuate pressureresponsive plate 1 to open damper ll partially or wholly depending uponthe amount of the static pressure. The amount of static pressure whichmust be exerted against 1 in order to obtain any desirable opening of Itis controlled by the angularity of lifter 20 with reference to the jetof air passing the opening l8. Further, as the extent of opening ofdamper ll increases, the weight of the arm and blade structure, I9, 20and 2|, causes a progressively increasing counter-weighting effectwhich, in combination with the paralleling of the blade 20 with the airflow, retards the tendency of the damper I4 to fly to full openposition.

Under certain conditions generally variable with the type of coal usedas fuel it is desirable to restrict the damper opening so that at notime may it be full open. The adjustable stop l2 controls this and maybe positioned at any desired point along curved slot It to so restrictthe amount of forced draft directed to the fuel bed.

The relative position of blade 20 to the air stream passing through l8determines its lifting power and under similar varying conditions itsangle may be changed to determine the extent of the preliminary openingof damper Hi. It is clear that if adjusted so as to be parallel to thedirection of the air stream it will not be affected by the air Jet butas the angle is increased the amount of its lifting movement will beincreased. The adjusting means 22 are accessible by removing housing 8.

,By the use of air passage-way l8 and closure plate l1 conduit 4 ispractically closed to air passage during "of! periods of forced draftfiring but thedamper I4 is almost immediately opened the correctdistance when on operation occurs due to the air jet I8 and strikinglifting blade 20. This feature is used in my invention for actuating aregulation of air fiow characteristics as well as for the purpose abovestated. Hence the invention consists in a combination of the followingelements: A by-pass to allow a close fitting damper, a nonvariablestatic pressure responsive element, a member regulable to producevariable auxiliary lifting and counter-weighting effects depending uponthe angularlty of its setting relative to an air jet and a quantitativeadjustment for air flows consisting of an adjustable stop limitingmaximum damper opening. The manner of combining, arranging and applyingthese elements as disclosed is changeable to secure similar effects andit is understood that my invention applies to the use of the elementsstated in any combination which secures qualitative and quantitative airadjustment in forced draft firing of solid fuels substantially asdescribed.

I claim:

1. In an automatic air control for use in forced draft firing comprisingan air conduit, a damper positioned in the conduit,'means responsive tostatic pressure for opening said damper, an air passageway from one sideof said damper to the other and a valve normally closing said passagewaybut adapted to open when forced draft is directed against the closeddamper, the improvement that comprises a lifting member operably curedto said damper and adapted to be struck by the air jet passing throughsaid air passageway and means for adjusting saidlifting member to varyits angle relative to the direction of the air jet to regulate itslifting effect.

3. In an automatic air control for use in forced draft firing comprisingan air conduit, a damper positioned in the conduit, means responsive tostatic pressure for opening said damper, and an orifice through saiddamper the improvement that comprises a blade positioned in the path ofair fiowing through said orifice, said blade being connected to saiddamper so as to exert a turning effect ,on said damper in the directionof its opening.

4. Automatic air control for use in forced draft firing comprising anair conduit, a damper positioned in the air conduit, means responsive tostatic pressure in the conduit for openingthe damper, an air passagewayleading from one side of the damper to the other, a valve normallyclosing said passageway but adapted to open under forced draft, alifting blade joined to said damper and normally positioned adjacent andopposing the air passageway and adapted to be moved by forced draftpassing therethrough, and adjustable means for limiting the opening ofsaid damper. v

5. Automatic air control for use in forced draft firing comprising anair conduit, a damper positioned in the air conduit, means responsive tostatic pressure in the conduit for opening the damper, an air passagewaythrough said damper,

